OpenStack “backing file” cleanup script

Sometimes OpenStack leaves the “backing file” of the sparse root filesystem behind after a VM is destroyed. Those files are located in /var/lib/nova/instances/_base/ and can use up hundreds of GB. To find out which of those backing files are orphans and delete them, someone has posted a script, which I slightly improved:

cd /var/lib/nova/instances
find -name "disk*" | xargs -n1 qemu-img info | grep backing | sed -e 's/.*file: //' -e 's/ .*//' | sort | uniq > /tmp/ignore
while read i; do
ARGS="$ARGS \( \! -path $i \) "
done < /tmp/ignore
eval "find /var/lib/nova/instances/_base/ -type f $ARGS -delete"

Someone in the original post claimed that adding remove_unused_base_images = True to nova.conf would do this automatically, however I could not reproduce that on Folsom.